3.1 Stress-strain curves
Figure 1 illustrates the stress-strain curves of thermal treatment granite under triaxial cyclic loading compression, it is in good agreement with the stress-strain curves of granite under monotonic loading except for granite specimen when T = 600°C,σ 3 = 0 MPa. It can be explained that cyclic loading has a slight effect on the crack propagation of intact granite. However, when more thermal cracks were induced in rock, the cracks were easier to propagate under uniaxial cyclic loading, and make the strength of specimen decreases. Confining pressure can restrain the crack propagation, therefore cyclic loading has a slight effect on the stress-strain curves of granite specimen when T = 600°C,σ 3 = 40 MPa.
At initial loading at each cycle, the axial loading and unloading stress-stain curves show a concave type. It means that fissures in granite opened under unloading process, and then reclosed under loading process. After the first loading-unloading compression, the concave characteristic is not evident, which indicates that the closed fissures cannot open completely. The concave type is more obvious after the peak strength, which indicates that the fissures induced by axial stress also opened and reclosed in the cyclic loading-unloading process. The loading enter to the elastic stage with increasing axial stress, even though the macro-crack has been formed, there is also elastic stage after the peak strength. The loading enter to the plastic stage after the elastic stage, the plastic stage is unobvious before the peak strength, and it is more obvious with increasing cycle number after the peak strength. After the peak strength, the axial stress decreases gradually with cycle number. However, the residual strength of specimen under the cyclic loading is larger than that under monotonic loading on the whole.